This invention relates to methods and apparatus for applying electrochemically active paste to a metallic grid or other substrate for use in electrochemical cells, and particularly relates to such use in lead-acid systems.
The usual type of conventional lead-acid pasting machine utilizes a substantially rigid grid (e.g., cast lead) which moves horizontally while being pasted with active material via an open-air vertical feed gear pump. This pump directly bears and provides a downward force upon the moving grid. The paste is injected into the grid interstices under a positive head of pressure. The pump is not designed to meter out a given flow rate or amount of paste, but rather discharges an excess of paste which is screeded off by a doctor blade positioned downstream of the pump, which blade bears directly upon the moving grid. Excess paste is recycled to the open-fed pump. One drawback with such machines is that the resultant thickness of the battery plate is determined solely by the thickness of the grid substrate. Oftentimes such a grid will consist of an expanded mesh which, because of irregularities in its surface, will vary in thickness along its length, thus producing a plate having a non-uniform thickness of pasted active material. Such conventional processes are conducted in a non-continuous fashion.
Furthermore, the open-fed aspect of such conventional machines requires close control to prevent air degradation of the paste, such paste being commonly known to set-up in a very short time when exposed to air. Another drawback with such machines has been their tendency to cause separation of the constituents of the paste as it is applied under pressure to the moving grid much like a calendaring operation, i.e., the aqueous vehicle and lead-oxide constituents of the paste may form distinct phases.
This conventional machine is particularly ineffective when the type of grid substrate utilized is relatively soft or deformable rather than of the conventional rigid plate type. The downward force produced by the pump mechanism on the moving grid, and the downward force produced by the doctor blade bearing upon the moving grid both have the tendency to deform and tear apart a relatively soft grid and prevent its continued free passage. Such soft, relatively pure lead grids have found important recent utility in lead-acid cells of the sealed, rechargeable, maintenance-free type operating on an oxygen cycle.
The primary object of the present invention is to provide a method and apparatus for continuously pasting a highly viscous, thixotropic battery paste onto a porous substrate, in which accurately predetermined plate dimensions are obtained. It is a further object to overcome the drawbacks presents in the prior art.